Calcite
Calcite
is a carbonate mineral
and the most stable polymorph of calcium
carbonate (CaCO3). The other polymorphs
are the minerals aragonite and vaterite.
Aragonite will change to calcite
at 380-470°C, and vaterite is even less
stable.
Properties
Calcite
crystals are trigonal-rhombohedral,
though actual calcite rhombohedra are
rare as natural crystals. However,
they show a remarkable variety of habits
including acute to obtuse rhombohedra,
tabular forms, prisms, or various
scalenohedra. Calcite exhibits several
twinning types adding to the variety of
observed forms. It may occur as
fibrous, granular, lamellar, or compact.
Cleavage is usually in three directions
parallel to the rhombohedron form. Its
fracture is conchoidal, but difficult
to obtain.
It
has a defining Mohs hardness
of 3, a specific gravity of 2.71, and
its luster is vitreous in crystallized
varieties. Color is white or none,
though shades of gray, red, orange,
yellow, green, blue, violet, brown, or
even black can occur when the mineral
is charged with impurities.
Calcite
is transparent to opaque
and may occasionally show
phosphorescence or fluorescence. A
transparent
variety called Iceland spar is used for
optical purposes. Acute scalenohedral
crystals are sometimes referred to as
"dogtooth spar" while the rhombohedral
form is sometimes referred to as
"nailhead spar".
Single
calcite crystals display
an optical property called birefringence
(double refraction). This strong
birefringence causes objects viewed
through a clear piece of calcite to
appear doubled. The birefringent effect
(using calcite) was first described
by the Danish scientist Rasmus Bartholin
in 1669. At a wavelength of ~590
nm calcite has ordinary and
extraordinary refractive indices of
1.658 and
1.486, respectively. Between 190 and
1700 nm, the ordinary refractive index
varies roughly between 1.6 and 1.4,
while the extraordinary refractive
index varies between 1.9 and 1.5.
Calcite,
like most carbonates,
will dissolve with most forms of acid.
Calcite can be either dissolved
by groundwater or precipitated by
groundwater, depending on several
factors
including the water temperature, pH, and
dissolved ion concentrations.
Although calcite is fairly insoluble in
cold water, acidity can cause
dissolution
of calcite and release of carbon dioxide
gas. Ambient carbon dioxide, due
to its acidity, has a slight
solubilizing effect on calcite. Calcite
exhibits
an unusual characteristic called
retrograde solubility in which it
becomes
less soluble in water as the temperature
increases. When conditions are
right for precipitation, calcite forms
mineral coatings that cement the
existing rock grains together or it can
fill fractures. When conditions
are right for dissolution, the removal
of calcite can dramatically increase
the porosity and permeability of the
rock, and if it continues for a long
period of time may result in the
formation of caves. On a landscape
scale,
continued dissolution of calcium
carbonate-rich rocks can lead to the
expansion
and eventual collapse of cave systems,
resulting in various forms of karst
topography.
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